I shot a roll of Tri-X with an AE-1. About one third of the frames had dark areas covering part of the image. I changed batteries and shot a test roll at various shutter speed and aperture combinations, which turned out fine. A few days later I ran another roll through the same camera. Again, dark areas vertically across about half the frames. I scanned a representative portion and it is attached. Now I am not sure whether this is a problem with the camera or the developing. Note how the dark (overexposed?) areas get progressively darker. On some frames the dark areas extend to the outer edges of the film and seem to correspond with the perforations. My developing practices are uniform, stainless steel reel and tank, Diafine at 75F for three and a half minutes. Any thoughts on what's causing this and how to correct it? My best guesses are light leak (but why not evident on all frames?) or shutter curtain hanging up.